As illustrated in FIG. 1, one can envision a satellite-based cellular radiotelephone infrastructure as consisting of a constellation of satellites in a low Earth, polar orbit, each satellite having a number of satellite-to-ground frequencies that illuminate hundreds to thousands of square miles of global surface area. Each frequency (F3, for example) could be reused (both from the very same satellite 102 as well as by neighboring satellites 101), provided sufficient geographic separation is maintained to avoid causing interference among radio-telephone subscribers (168) on the ground. A subscriber (168) is handed off from frequency F3 to frequency F1 and from satellite (102) to satellite (101) as the constellation moves overhead. Using switch exchanges aboard the satellite, the satellites route calls among themselves over high speed, high bandwidth inter-satellite links (160, 161, 162) in order to globally connect conversants on the ground (168 and 170). A more complete presentation of such a satellite cellular system is given in U.S. patent application Ser. No. 263,849 to Bertiger, Satellite Cellular Telephone and Data Communication System, filed 28 Oct. 1988 assigned to Motorola.
Since the surface area illuminated by a single frequency is so large compared to terrestrial cellular radio-telephone systems, satellite cellular systems have far lower subscriber capacity and find limited utility in low density and rural markets; metropolitan cellular traffic is far too dense and demands much higher spectral reuse efficiency. With the high cost of building, launching, operating and maintaining a satellite infrastructure, the economic viability of offering radiotelephone service via satellite depends on the ability to integrate terrestrial radio-telephone networks into satellite cellular systems.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.